1. Field of the Invention
This invention relates to a new method of electronic imaging and its applications. The invention particularly relates to a new imaging method of focal plane arrays (FPA), in which the image signal is modulated while the background and/or dark current are not modulated, and the image signal integration is correlated to its modulation.
2. Description of Related Art
By using intersubband transition, the quantum well infrared photodetectors (QWIP) operating in photoconductor (PC) mode are made of wide-bandgap materials, such as GaAs/AlGaAs. The maturity of GaAs material growth and its processing technology places the QWIP in a strategically important position. Large-sized (512xc3x97512 or greater), multi-color, reliable, reproducible, and low-cost QWIP FPAs working in the important 8-14xcexc long wavelength infrared (LWIR) atmospheric window seem to be within reach for military, scientific, and commercial applications. However, the effort to develop a competitive LWIR QWIP FPA is still being challenged by QWIP""s one impediment: its large dark current. In order to get significant IR response, the PC mode QWIP must be biased at a few volts, which leads to a large dark current due to electron thermal emission, thus making it difficult for its use at T greater than 60 K when the wavelength xcex greater than 8xcexc. Although the 8-12xcexc QWIP""s dark current has been reduced and responsivity increased through innovative designs, it still cannot operate at the convenient liquid nitrogen temperature of 77 K to perform the critically important 8-12 micron thermal imaging.
Note that the condition of
Id or Ib greater than  greater than Isxe2x80x83xe2x80x83(1)
exists in other cases, too. In a recent NASA research project, the 15xcexc QWIP""s dark current (working at 55 K) Id is three orders greater than the scene or signal current Is. In solar magnetography, the background photocurrent Ib is more than four orders greater than Is. In long wavelength IR spectroscopy and spectroscopic imaging, the narrow bandwidth of signal mandates a low ratio of Is/Id. All these three cases, as well as other scientific and biomedical imaging and spectroscopic applications involving weak signals, have the same feature as the QWIP FPA. The conventional staring array imaging method cannot perform satisfactorily, since its signal integration time is limited by the predominant dark or background current. As the background current Ib plays an equivalent role as the dark current Id, only Id is used in the following discussion. Since imaging and spectroscopy share the same signal integration and readout technology, we use the terminology of FPA imaging, which can be applied to both cases.
To deal with the difficulties of imaging a weak signal with presence of a strong background or dark current, the concept of current memory background subtraction (CMBS) was proposed by the Jet Propulsion Laboratory (JPL) Group of Advanced Imager and Focal Plane Technology. The photodetector""s large dark current is subtracted by a current sink, which was memorized during the FPA""s calibration phase. The result is an increase of effective dynamic range by one order, albeit at the cost of increased shot noise.
Note that in the case of a single photodetector recovering weak optical signal from strong noise, the method of lock-in amplification was invented. In lock-in amplification, a weak signal is modulated together with a strong noise, which mainly consists of the white shot noise and the 1/f flick noise. After going through an amplifier with a bandwidth centered at the modulation frequency, and a band pass filter to maintain the modulated signal while eliminating unwanted noises at frequencies different from the modulation frequency, the modulated signal is fed to a phase sensitive detector (PSD). As a unit square wave multiplier in phase with the modulator, the PSD feeds a DC component, which is proportional to the modulated signal amplitude, to a low pass filter. When the noise is pure white noise, the resulting improvement of the signal to noise ratio due to lock-in amplification is                                                         (                              S                /                N                            )                        o                                              (                              S                /                N                            )                        i                          =                                            B              LPF                                      B              WN                                                          (        2        )            
where (S/N)i is the input signal to noise ratio, (S/N)o the output signal to noise ratio, BLPF the bandwidth of the low pass filter, and BWN the bandwidth of the original white noise.
It is interesting to note that although the lock-in amplification and the CMI presented in this invention both utilize the modulation of signal, the two methods are different. The lock-in amplifier works only for a single detector, while the correlated modulation imaging technique does for an array of detectors (one-dimensional linear array or two-dimensional -area array). The single photodetector with lock-in amplification operates continuously, with a phase sensitive detector and a low pass filter as the key components. In the method of CMI, the improvement of signal to noise ratio is achieved due to the modulation of signal and correlated multi-cycle integration of the signal, with the increase of total integration time as the key factor.
Is DC image signal photocurrent generated in the detector by the scene of imaging. Is is defined as the average of the real image signal current is, which is time varying, through its integration cycle.
Id DC dark current thermally generated. Id is defined as the average of the time-varying real dark current id.
Ib DC background current, which is generated by photons of the same wavelengths under detection. As Is and Id, Ib is defined as the average of the time varying real background current ib. When Ib can be modulated separately from signal current Is, the background current Ib plays the same role as the dark current Id. In this invention, we treat the two as equivalent, using Id to denote Ib+Id.
Ti Integration time. For CMI, Ti=mxcfx84, where m is the number of integration cycles of the CMI unit pre-amplifier, and xcfx84 the period of each cycle.
R Signal to noise ratio, defined as       R    =                  Signal        /        Noise            =                        N          s                                                    n              2                        _                                ,
xe2x80x83where Ns is the number of electrons due to the signal current Is, and {overscore (n2)} is the root mean square value of the total number of electrons due to random noise.
D Dynamic range in decibels, defined as D=20 log10 Rmax, when the maximum integration time is utilized.
f Frequency.
a) Angular frequency. xcfx89=2xcfx80f
xcfx89m Modulation frequency. xcfx89m=2xcfx80fm=2xcfx80/xcfx84.
T(xcfx89) Noise transmission function of a conventional single cycle integrator for a conventional FPA
T(xcfx89)={square root over (Vo(xcfx89,xcfx86)Vo*(xcfx89,xcfx86))}, where Vo(xcfx89,xcfx86) is the output voltage of the integrator with a unit harmonic current i (t)=ej(xcfx89t+xcfx86) as the input.
Tm (xcfx89) Noise transmission function of a CMI unit pre-amplifier version 1 for a CMI FPA